The present invention relates generally to the art of forming thin films, and more particularly to improvements in the magnetron sputtering process and apparatus for forming such films which may be highly permeable materials.
One technique for depositing thin films of a desired material on a substrate is diode sputtering. A target comprising the material to be deposited, is bombarded by gas ions which have been accelerated by an intense electric field. The bombardment ejects atomic sized particles of the target which settle upon the substrate surface as a thin film. This sputtering process is slow compared to other techniques and the electric voltage required to produce a diode sputtered film is relatively high. The current saturates at a low value.
Disadvantages associated with the diode sputtering process have been alleviated to a large degree by the use of magnetron sputtering. As can be seen in FIG. 1A, an array of magnets 10 and 12 is positioned behind a low permeability target material 14 where the magnetron may produce a discharge of "racetrack" shape and where the magnets may be of the type disclosed in U.S. Pat. Nos. 4,162,954, 4,180,450 and 4,265,729, issued to Charles F. Morrison, Jr., which patents are incorporated herein by reference. Coupling plate 16 serves to short the magnetic fields between the two magnets at the lower portion thereof. Because of the low permeability of the target material, the magnetic lines of force 18 extend from the magnets and pass through the target material 14 and travel substantially parallel to the plane of the target surface for a certain distance. An electric field is established perpendicular to at least a portion of the magnetic field. Gas ions are accelerated by the electric field and strike target 14 causing it to eject atomic sized particles as in diode sputtering. However, the magnetic field above the target surface confines secondary electrons ejected from the target to the vicinity of the target surface and thus accelerates the rate of collisions between the secondary electrons and gas molecules of the gas plasma (generally argon). These additional collisions serve to generate additional gas ions and, hence, more gas plasma which is confined to the vicinity of the target surface. Thus, the deposition rate of magnetron sputtering over that of diode sputtering is increased by an order of magnitude.
It can be seen that the looping magnetic field as indicated by lines of force 18 is necessary to trap the plasma near the surface of target 14. However, if it is desirable to sputter a high permeability material with magnetron sputtering, the looping magnetic field will be short circuited as shown in FIG. 1B. Effectively the high permeability target 24 couples all of the magnetic lines of force from one magnet to the other just as does the coupling plate 16. The lack of the looping magnetic field 18 to trap the plasma in the vicinity of the high permeability target material would reduce the magnetron sputtering to that of ordinary diode sputtering with its attendant relatively slow sputter rate due to current saturation.
A number of solutions have been attempted to obtain magnetron sputtering of highly permeable materials with only limited success. In one embodiment, a very thin high permeability target is utilized so as to become saturated by the magnets and thus incapable of shunting all of the magnetic field. Unfortunately, if the targets are made thin enough such that the magnets do not shunt virtually all of the field, the targets are rapidly depleted before a film is accumulated on substantial quantities of receiving substrate. Other approaches are to utilize relatively normal target thicknesses but in conjunction with high strength magnets again serving to saturate the target material and maintain a weak magnetic field looping thereover. This generally requires at least a second set of magnets or an extremely powerful electromagnet. This works reasonably well with moderate sized targets of iron and nickel but is generally inadequate for Permalloy, samarium cobalt, and other very high permeability materials. This approach is generally described in my co-pending patent application Ser. No. 28,434, filed Apr. 9, 1979.
One further method of permitting magnetron sputtering is to reduce the strength of field required to saturate the target material. This can be accomplished by heating the target material to above its Curie point and this is discussed in U.S. Pat. No. 4,299,678 issued to Meckel on Nov. 10, 1981. However, none of the above methods lend themselves to serious industrial coating and thus most sputtering of highly permeable materials is still done by diode sputtering with its very low rates of accumulation.